The adverse effect of high-energy electromagnetic (EM) fields incident on communication radios has been known for a long time. The usual protection against such high field levels has either been fuses, spark gaps or component circuit breakers. Fuses, once activated, need to be replaced before the radio can operate again. Component circuit breakers such as semiconductor diodes or capacitive shunts and spark gaps are limited in the amount of current/voltage that can be shunted and by their reaction time. This disclosure describes a technique to make the front-end of a radio system tolerant of high-energy EM fields while also having high sensitivity to low-energy RF EM signals. One known way to sense incident EM radiation is to use an antenna that is electrically coupled to an electro-optic modulator. The electro-optic modulator is part of an RF-photonic link that also includes a laser light source and a photodetector. The electro-optic modulator modulates the intensity or phase of the light supplied to it from the laser according to the amplitude of the RF signal coupled to that modulator from the antenna. The modulated light from the electro-optic modulator is then supplied to the photodetector which converts that modulated light into an RF electrical output signal. One need of this approach is to achieve strong depth of modulation and good sensitivity to weak RF input signals.
A prior art technique for an electro-optic modulator uses a single optical waveguide formed in electro-optic material and an array of multiple modulator sections that are optically connected in series to increase the depth of modulation, as described by: James H. Schaffner and William B. Bridges, “Broad Band, Low Power Electro-Optic Modulator Apparatus and Method with Segmented Electrodes,” U.S. Pat. No. 5,291,565, Mar. 1, 1994. Each of the multiple modulator sections is driven by its own set of electrodes. This prior art disclosure uses printed circuit electrodes and a printed circuit delay structure to feed the input electrical signal to those electrodes such that a phase match is maintained between the RF signal and the optical signal at each electrode. These printed circuit electrodes, because of the high level of their fringing field, cannot withstand a high power RF input. In contrast, the present disclosure uses RF waveguides instead of printed circuit electrodes and printed circuit delay and feeding structure to supply and delay the RF drive signal to the modulators, in order to avoid damage from an incident high power electromagnetic pulse.
Another prior art electro-optic modulator with multiple electrodes that drive an optically series connection of modulator sections is described by: William B. Bridges, “Antenna-Fed Electro-Optic Modulator,” U.S. Pat. No. 5,076,655, Dec. 31, 1991. The multiple electrodes are electrically fed by means of the EM field propagating in an RF waveguide, with those multiple electrodes acting as multiple antenna elements that couple the EM field propagating in the RF waveguide. Those multiple electrodes of the modulator are physically separate from the metal walls or enclosure of the RF waveguide and no electrical current flows directly from the metal walls of the RF waveguide to those electrodes. In contrast to this prior art, the present disclosure uses modulators whose electrodes are physical and electrical extensions of the metal walls of the RF waveguide. Electrical current can flow directly from those metal walls to those modulator electrodes. Thus, unlike the prior apparatus of Bridges, the presently disclosed modulators do not have electrodes that are separate from the metal walls of the RF waveguide that feeds the RF signal to those modulators.
An example of arrays of electro-optic modulators whose electrodes also act as antennas is described by: Joseph E. Moran, “Apparatus and System for Imaging Radio Frequency Electromagnetic Signals,” U.S. Pat. No. 6,703,596, Mar. 9, 2004. This patent is for an imaging antenna array wherein each antenna element in the antenna array is connected to a separate electro-optic modulator. The apparatus of Moran uses multiple antenna elements and has a single modulator electrically coupled to each antenna element and physically located adjacent to that antenna element. Each antenna element also serves as a drive electrode for a modulator. In contrast, the present disclosure has multiple electro-optic modulators or modulator sections electrically coupled to each antenna element. These modulators or modulator sections can be located at some distance away from the antenna element, being coupled to the antenna element by means of RF waveguides.
The use of multiple parallel plate (TEM) RF waveguides in an array, as a multi-furcation of space, has been known for a long time. However, these prior art have been used for electric-field combining, for example in RF lenses. The difference between these prior art and the use of parallel plate RF waveguides in the present disclosure is that our parallel plate multi-furcation is used to feed RF voltage to an array of optical modulators. These prior art do not include this RF to optical conversion and do not include any optical modulators. The classical paper on the parallel plate lens is: J. Ruze, “Wide-Angle Metal-Plate Optics,” Proceedings of the I.R.E., Vol. 38, No. 1, January 1950, pp. 53-59.
The prior art also includes the following documents which are referenced herein:    1a. NAVSYNC CW20 GPS receiver specification—www.navsync.com    1b. LINX Technologies RXM-900-HP-II RF Module specification—www.linxtechnologies.com    1c. MAXIM, “Receiver Sensitivity Equation for Spread Spectrum Systems, MAXIM application note 1140, Jun. 28, 2002, www.maxim-ic.com/an1140.    2. Dr. Lowell Wood, acting chairman for the Commission to Assess the Threat to the U.S. from Electromagnetic Pulse Attack, “Opening Statement before the United States Senate Committee on the Judiciary, Subcommittee on Terrorism, Technology and Homeland Security”, Mar. 8, 2005.    3. R. T. Lee and G. S. Smith, “A Design Study for the Basic TEM Horn Antenna,” IEEE Antennas and Propagation Magazine, Vol. 46, No. 1, February 2004, pp. 86-92.    4. A. K. Ghatak and K. Thyangarajan, Optical Electronics, Cambridge University Press, Cambridge, 1989, pp. 441-447.    5. Emerson and Cuming Microwave Products, Eccostock® HiK500F data sheet, www.eccorsorb.com, rev. May 11, 2007.    6. G. E. Betts, L. M. Johnson, and C. H. Cox, “High-Sensitivity Bandpass RF Modulator in LiNbO3,” SPIE Integrated Optical Circuit Engineering VI, Vol. 993, 1988, pp. 110-116.    7. J. W. Shi, C. A. Shiao, Y. S. Wu, F. H. Huang, S. H. Chen, Y. T. Tsai, and J. I. Chyi, “Demonstration of a Dual-Depletion-Region Electroabsorption modulator at 1.55-μm Wavelength for High-Speed and Low-Driving-Voltage Performance,” IEEE Photon. Technol. Lett., Vol. 17, No. 10, October 2005, pp. 2068-2070.    8. S. B. Cohn, “Optimum Design of Stepped Transmission-Line Transformers,” IRE Trans. Microwave Theory Tech., Vol. 3, No. 3, April 1955, pp. 16-20.    9. K. Morito, S. Tanaka, S. Tomabechi, and A. Kurmata, “A Broad-Band MQW Semiconductor Optical Amplifier with High Saturation Output Power and Low Noise Figure,” IEEE Photon. Technol. Lett., Vol. 17, No. 5, May 2005, pp. 974-976.
Many of today's sophisticated communication radio receivers are extremely sensitive and need to demodulate signals that are well below −100 dBm, that is, less than 100 fW (see documents 1a-1c mentioned above). While this low signal threshold increases the range between the transmitter and the receiver, it also makes these receivers highly susceptible to destruction by high power incident radiation. These high power incident radiation can be caused on purpose (see document 2 mentioned above), or could even be accidental (e.g. from crossing the path of a high power microwave beam). The invention described in this disclosure addresses the need to maintain high radio sensitivity, while at the same time insuring tolerance to transient high power electromagnetic radiation.
In order to understand the ability of the disclosed receiver front-end assembly's capability to withstand high power microwave or RF radiation while maintaining high sensitivity, consider the single element 10 shown in FIG. 1. This element is described in greater detail in the related U.S. Ser. No. 12/176,071, filed on the same date as this application and entitled “Microwave receiver front-end assembly and array” mentioned above, but is summarized here as background material for a better understanding of the present invention. A transverse electromagnetic (TEM) horn antenna 50 channels the RF signal into a TEM waveguide 25 in which one or more electro-optic modulators 20 are located. Although the horn antenna 50 needs not be TEM, it is important that the waveguide 25 be TEM in order to establish as uniform a transverse electric field as possible across an electro-optic modulator 20 embedded in the waveguide 25. An optical signal from a laser (not shown) interacts with the RF signal inside the width of the TEM waveguide 25 (see document 4 noted above) resulting in modulation of the phase or intensity of that optical signal. Other antennas besides horn antennas could be used as long as they have appropriate transitions to the TEM waveguide 25. The waveguide 25 is preferably filled with a dielectric material whose relative permittivity (or dielectric constant) has a value that is close to the permittivity of the material from which the electro-optic modulator 20 is fabricated. For example, if the modulator 20 is fabricated from lithium niobate (LiNbO3), the required dielectric constant should be approximately thirty. This can be achieved using ceramic based material such as Emerson and Cumings Eccostock (see document 5 noted above). The reason for this constraint is so that there is little reflection at the interface between the RF waveguide and the electro-optic modulator(s). The TEM waveguide 25 may be terminated in a high power load or another TEM waveguide 26 or TEM horn 70, or may even not be terminated.
The electro-optic modulators 20 have integrated optic waveguides, but have no printed circuit electrodes, which makes them quite a bit different than other prior art integrated optic modulators, such as those described in document 6 noted above. This avoidance of printed circuit electrodes is to prevent the electric field across an individual integrated optic waveguide from becoming too high during exposure to a high power electromagnetic pulse, which could produce a fringing electric field so high as to cause the dielectric material nearby the integrated optic waveguide to breakdown. With the assembly of FIG. 1, the electric field is maintained between the two pieces of metal that make up the top and bottom conductors 53 of the TEM waveguide. Because the depth of modulation of the optical signal in the electro-optic modulator is directly proportional to the electric field strength, the modulation is weakened by the need to keep the top and bottom conductors of the TEM waveguide far apart. Sufficient distance must be kept between those top and bottom conductors to avoid breakdown of the dielectric fill material and of the electro-optic material when that assembly is exposed to the high power electromagnetic pulse. Note that the fringing fields produced in a TEM waveguide can be significantly lower than the fringing fields produced at a printed circuit electrode.
In order to improve the sensitivity of the front-end assembly to weak input electromagnetic signals one can use some combination of multiple electro-optic modulators. One way to combine multiple modulators is to cascade those modulators, as described in related U.S. patent application Ser. No. 12/141,825, filed on Jun. 18, 2008 and entitled “Optoelectronic modulator and electric-field sensor with multiple optical-waveguide gratings”. Another way to combine multiple modulators is to arrange them optically in parallel but feed them from the antenna electrically in series, as described in related U.S. patent application Ser. No. 12,176,089, filed on the same date as this application and entitled “Parallel Modulator Photonic Link”. The optical outputs from these multiple parallel-arranged modulators are combined to produce a stronger output signal. The approaches described in these two related patent applications make use of multiple modulators that are formed within and electrically coupled to the same TEM RF waveguide. In contrast, the multiple modulators disclosed herein are electrically coupled to different RF waveguide portions of the multi-furcation. Those multiple modulators also may be formed exterior to the RF waveguide portions and attached to the ends of those RF waveguide portions. Preferably, those multiple modulators are formed on the same electro-optic modulator substrate.